The present invention relates to light emitting diode (LED) devices and, more particularly, to an LED device that performs phosphor conversion on all of the primary radiation emitted by the light emitting structure of the LED device to produce white light.
With the development of efficient LED devices that emit bluish or ultraviolet (UV) light, it has become feasible to produce LED devices that generate white light through phosphor conversion of a portion of the primary radiation emission of the light emitting structure of the LED device to longer wavelengths. Conversion of primary emission to longer wavelengths is commonly referred to as down-conversion of the primary emission. An unconverted portion of the primary emission combines with the light of longer wavelength to produce white light. LED devices that produce white light through phosphor conversion are useful for signaling and illumination purposes. LED devices having light emitting structures that emit white light directly currently do not exist.
Currently, state-of-the-art phosphor conversion of a portion of the primary emission of the LED devices is attained by placing phosphors in an epoxy that is used to fill a reflector cup, which houses the LED device within the LED lamp. The phosphor is comprised as a powder that is mixed into the epoxy prior to curing the epoxy. The uncured epoxy slurry containing the phosphor powder is then deposited onto the LED device and is subsequently cured.
The phosphor particles within the cured epoxy generally are randomly oriented and interspersed throughout the epoxy. A portion of the primary light emitted by the LED device passes through the epoxy without impinging on the phosphor particles, whereas a portion of the primary light emitted by the LED device impinges on the phosphor particles, thereby causing the phosphor particles to emit yellowish light. The combination of the primary bluish light and the phosphor-emitted yellowish light produces white light.
One disadvantage of using phosphor-converting epoxy in this manner is that uniformity in the white light emitted by the LED device is difficult, if not impossible, to obtain. This non-uniformity is caused by non-uniformity in the sizes of the phosphor particles mixed into the epoxy slurry. Currently, phosphor powders having uniform phosphor particle sizes generally are not available. When the phosphor powder is mixed into the epoxy slurry, the larger phosphor particles sink faster than the smaller phosphor particles. This non-uniformity in spatial distribution of the phosphor particles exists in the epoxy once it has been cured.
Therefore, obtaining a uniform distribution of the phosphor particles within the epoxy is very difficult, if not impossible, due to the non-uniformity of the sizes of the phosphor particles. This inability to control the sizes of the phosphor particles and their locations within the epoxy results in difficulties in controlling the fraction of the primary light that is summed with the phosphor-emitted yellowish light to produce white light.
Since this fraction cannot be precisely controlled, the quality of the white light produced by LED lamps may vary from one lamp to another, even for a given model manufactured by a particular manufacturer. Another disadvantage of this type of LED device is that the light emitting structure of the LED device is most efficient at emitting bluish light in the range of about 450 nanometers (nm) to about 500 nm. There is reason to believe that LED devices may be developed in the future that will operate efficiently at shorter wavelengths, e.g., between about 400 and 450 nm. It would be desirable to provide an LED device that is capable of producing primary light at these shorter wavelengths and of performing phosphor conversion on the primary light to produce white light. However, mixing primary light of wavelengths below 460 nm with the phosphor-converted emission will not produce white light due to the fact that the wavelengths of the primary emission are hardly visible.
Accordingly, a need exists for an LED device that is capable of producing high quality white light through phosphor conversion of all of the primary light, and that is capable of being reproduced in such a manner that the quality and uniformity of the white light generated by the LED devices is predictable and controllable.
The present invention provides an LED device that is capable of performing phosphor conversion on substantially all of the primary light emitted by the light emitting structure of the LED device to produce white light. The LED device comprises at least one phosphor-converting element located to receive and absorb substantially all of the primary light emitted by the light-emitting structure. The phosphor-converting element emits secondary light at second and third wavelengths that combine to produce white light. Typically, the second wavelength is greater than the first wavelength and the third wavelength is greater than the second wavelength. Secondary light at additional wavelengths may also be emitted by the phosphor-converting element. These additional wavelengths would also combine with the light of the second and third wavelengths to produce white light.
The phosphor-converting element generates the secondary light at the third wavelength in response to excitation by the primary light and/or the secondary light at the second wavelength. The excitation by the secondary light at the second wavelength is effected by macroscopic absorption and/or quantum-mechanical transfer. The phosphor-converting element includes either (a) a first host material doped with a first dopant and a second host material doped with a second dopant, which may or may not be the same as the first dopant, or (b) a host material doped with a first dopant and a second dopant. The first dopant emits the secondary light at the second wavelength and the second dopant emits the secondary light at the third wavelength. Furthermore, additional host materials and/or additional dopants that emit additional wavelengths of secondary light may be incorporated into the phosphor-converting element. These additional wavelengths of secondary light would be emitted by the phosphor-converting element in response to excitation by secondary light of the second or third wavelengths, or in response to excitation by the secondary light of one or more of the additional wavelengths. Secondary light of these wavelengths would then combine to create white light.
In accordance with an alternative embodiment, the light generated by the phosphor-converting element includes light of at least one additional wavelength, which is generated in response to excitation by (a) the primary light and/or (b) the secondary light at any wavelength shorter than the additional wavelength. The phosphor-converting element generates the secondary light at the additional wavelength in response to excitation by (a) the primary light at the first wavelength, (b) the secondary light at the second wavelength, and/or (c) the secondary light at the third wavelength. Excitation by the secondary light occurs by (a) macroscopic absorption and/or (b) quantum-mechanical transfer. The secondary light of the second, third and the additional wavelengths combines to produce white light.
The present invention is not limited with respect to the types of phosphor-converting elements that are utilized in the LED device, or with respect to the composition of the phosphor-converting elements. The host material includes a phosphor compound that is capable of incorporating an atomically-dispersed dopant. The dopant must have a particular chemical relationship to the host that makes it suitable for being incorporated into the host. The host materials comprising the phosphor-converting elements may be, for example, phosphor-converting powders in an encapsulant (e.g., epoxy), phosphor-converting organic dyes, phosphor-converting substrates, phosphor-converting thin films, closely-packed phosphor powder layers, etc. Also, various combinations of these host materials and/or of the phosphor-converting elements can be created. In one case, an epoxy encapsulant is mixed with a phosphor powder. The phosphor-converting element may comprise a second host material that is a phosphor-converting thin film. In all cases, all of the primary light will be absorbed by one or more of the phosphor-converting elements such that virtually no primary light is left unused.
Other features and advantages of the present invention and variations thereof will become apparent from the following description, drawings and claims.